French patent FR 2 802 724 recalls that it is known to make an inner rotor having permanent magnets presenting faces that face towards the stator that are convex, and a stator having teeth presenting end faces that are concave towards the rotor.
That prior patent describes a rotary machine in which the end faces of the teeth are convex towards the rotor and the magnets present faces facing towards the stator that are cylindrical about the axis of rotation.
European patent application EP 1 349 261 discloses a motor for an elevator in which a stator has teeth provided with pole enlargements (or “pole shoes”) and a rotor having permanent magnets.
It is desirable for the electric motors used for driving elevator cabins to generate as little vibration as possible, since the vibration is likely to be transmitted to the cabin, where it can disturb the comfort of passengers.
There exists a demand for reducing the overall size of motors, e.g. in order to make elevators easier to install.
A solution for achieving that object consists firstly in using motors without gearing or with a relatively small stepdown ratio, and secondly in causing the rotor to rotate more quickly.
Unfortunately, increasing the nominal speed increases the risk of encountering resonant frequencies both when operating at nominal speed and during starting and deceleration stages.